(1) Field of the Invention
The present invention relates to a process for preparation of low molecular weight polyols from high molecular weight polyols in a hydrogenolysis reaction under elevated temperature and hydrogen pressure which comprises providing in a reaction mixture the polyols, a base, and a metal catalyst prepared by depositing a transition metal salt on an inert support, reducing the metal salt to the metal with hydrogen, and passivating the metal with oxygen, and wherein the catalyst is reduced with hydrogen prior to the reaction. In particular, the process relates to the preparation of glycerol, propylene glycol, and ethylene glycol from sugar alcohols such as sorbitol or xylitol. In a preferred process, the catalyst comprises ruthenium deposited on an alumina, titania, or carbon support, and the dispersion of the ruthenium on the support increases during the hydrogenolysis reaction.
(2) Description of Related Art
Hydrogenolysis processes for cleavage of C--C and C--O bonds was first applied to oxygenated organic compounds in the 1930s. Since then, the majority of publications and patents relating to hydrogenolysis have focused on C6 sugar alcohols such as glucose and sorbitol as substrates for hydrogenolysis. In contrast, very few studies on hydrogenolysis have been concerned with C5 substrates such as xylose and xylitol.
In the prior art, conditions for sorbitol hydrogenolysis have involved a supported metal catalyst, a base promoter such as CaO or KOH, temperatures of about 180.degree. C. to 250.degree. C., and elevated hydrogen pressures (4 to 20 MPa). In early work, Ni/kieselguhr catalysts were shown to achieve 40% by weight yields of glycerol, 17% propylene glycol, and 16% ethylene glycol (Clark, Ind. and Eng. Chem. 50: 1125 (1958)). Propylene glycol and ethylene glycol were formed via secondary reactions on glycerol indicating that the C3-C4 bond cleavage was the primary mechanism in the hydrogenolysis of sorbitol. When Van Ling and Vlugter (J. Appl. Chem. 19: 43 (1969)) used ceria-promoted copper on silica catalysts, C3-C4 sorbitol selectivities of 65% to 70% were achieved. Using a Ru/C catalyst and sucrose as the reactant, Muller et al. in Heterogeneous Catalysis and Fine Chemicals II (M. Guisnet et al., eds.) Elsevier Press, Amsterdam, Netherlands, p 237 (1991) achieved a propylene glycol yield of 46% with 55% selectivity. Further, Montassier et al. (J. Mole. Catal. 70: 99 (1991)) were able to convert sorbitol and xylitol to desired products over both ruthenium on carbon support catalysts and sulfur-modified ruthenium on carbon support catalysts. They were able to obtain selectivities of 64% for C3-C4 cleavages of sorbitol, and 86% for C2-C3 cleavages of xylitol.
A number of patents have been issued regarding catalysts and processes for sorbitol and xylitol hydrogenolysis. Most noteworthy are U.S. Pat. No. 4,430,253 to Dubeck wherein S-modified (sulfided) ruthenium on carbon catalysts were disclosed in which 96% of the carbon in sorbitol was converted to propylene glycol, ethylene glycol, and glycerol; U.S. Pat. Nos. 4,401,823 and 4,496,780 to Arena wherein ruthenium/barium oxide on Al.sub.2 O.sub.3 catalysts were disclosed which achieved a 75% selectivity to propylene glycol and glycerol from sorbitol; U.S. Pat. No. 5,210,335 to Schuster et al. wherein Co/Cu/MnO.sub.x mixed metal oxide catalysts produced propylene glycol yields of 50 to 65%; and, U.S. Pat. No. 5,600,028 to Gubitosa et al. which disclosed a method for producing lower polyhydric alcohols such as glycerol from higher polyhydric alcohols such as sorbitol using a catalyst consisting of ruthenium on activated carbon with a BET surface area from 600 to 1,000 m.sup.2 per gram.
Other U.S. patents that disclose catalysts for hydrogenolysis reactions are U.S. Pat. No. 1,963,997 to Larchar and U.S. Pat. No. 2,004,135 to Rothrock which disclosed a Ni--CrO.sub.x catalyst that was useful for converting sorbitol to polyethylene glycol; U.S. Pat. Nos. 2,852,570 and 3,030,429 to Conradin et al. which disclosed a hydrogenolysis reaction using Cu-MgO-Ni catalysts in 16 MPa H.sub.2 at 220.degree. C. that yielded 35% glycerol and 30% ethylene glycol from sorbitol; U.S. Pat. No. 3,396,199 to Kasehagen which disclosed a hydrogenolysis reaction using a Ni on diatomaceous earth catalyst in 13 MPa H.sub.2 with an overall 69% conversion of sorbitol with 60% selectivity to glycerol; U.S. Pat. Nos. 4,338,472, 4,366,332, and 4,380,675 to Chao et al. which disclosed hydrogenolysis reactions using a Ni/SiO.sub.2 catalyst with a CaO promoter which produced yields from sorbitol of 27% glycerol, 25% propylene glycol, and 19% ethylene glycol; U.S. Pat. No. 4,404,411 to Tanikella which disclosed a hydrogenolysis reaction conducted in a batch reactor at 13 to 40 MPa hydrogen pressure that yielded 35% ethylene glycol, 40% propylene glycol, and 4% glycerol from xylitol using a Ni on SiAlO.sub.x catalyst in methanol and an alkali alkoxide promoter; U.S. Pat. No. 5,026,927 to Andrews et al. which disclosed a hydrogenolysis reaction that yielded 36% glycerol and 8% ethylene glycol from sorbitol using a H.sub.2 Ru(PPh.sub.3).sub.4 with KOH in NMP; and, U.S. Pat. Nos. 5,326,912 and 5,354,914 to Gubitosa et al. which disclosed ruthenium-based catalysts such as Ru/Cu/Pd/Pt on carbon and Ru/Sn on carbon that yielded 48% propylene glycol, 18% ethylene glycol, and 6% glycerol from sorbitol.
In general, the prior art processes use catalysts that require special handling procedures to prevent destruction of the catalyst. Furthermore, these catalysts have a relatively short useful life which limits the duration of the hydrogenolysis. Therefore, there is a need for a process that uses a catalyst which can be stored under atmospheric conditions, has a longer useful life during the hydrogenolysis reaction, and improves the overall selectivity of hydrogenolysis to desired products under mild conditions.